Revision of <i>Sagenopteris</i> (Caytoniales): a major lineage of the Mesozoic seed plants

Abstract Sagenopteris is the most common fossil organ attributed to Caytoniales and is represented by compound leaves having several (usually four) lanceolate leaflets in pseudo‐palmate arrangement. Since Presl's description of Sagenopteris in 1838, at least 82 species have been identified worldwide. However, most were established on a few isolated leaflets without sufficient details for confident segregation from other forms. After a global evaluation of the character states, nomenclature and taxonomic validity of all described Sagenopteris species, only five can be retained as nomenclaturally valid and morphologically distinct: Sagenopteris acuminata (lectotype species), S. colpodes , S. hallei , S. phillipsii and S. pualensis . Comprehensive assessment of Sagenopteris foliage based on historical studies and institutional collections shows a high degree of morphological plasticity within individual leaf populations. Multivariate plots of key leaf characters show broad morphotypes within populations that, if selectively sampled, might readily result in the recognition of multiple fossil‐species. Cuticular features are more stable for species‐level differentiation. Temporo‐spatial analysis of Sagenopteris suggests it originated in the European (palaeo‐subtropical) region by the Anisian (Middle Triassic) and underwent significant range expansion during the Late Triassic. Surviving the end‐Triassic mass extinction, Sagenopteris quickly became one of the dominant gymnosperms in the aftermath of the biotic crisis, reaching its peak abundance during the Early and Middle Jurassic. This taxon began to decline in floristic importance from the Late Jurassic to Late Cretaceous. The youngest occurrences of Sagenopteris , globally, are from Cenomanian–Campanian assemblages of middle to high northern latitudes. No examples are known from the Cenozoic.